Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration. The process is done by applying a pressure in excess of the osmotic pressure. Reverse osmosis has numerous applications, including, but not limited to, the separation of pure water from seawater and brackish waters where seawater or brackish water is pressurized against one surface of the membrane, causing transport of salt-depleted water across the membrane and emergence of potable drinking water from the low-pressure side. Other applications of reverse osmosis include concentrating food liquids, e.g., orange juice, production of maple syrup, and production of hydrogen.
There are typically two types of semipermeable membrane configurations used for reverse osmosis: spiral wound cartridges and hollow core. The membranes have a dense layer in the polymer matrix where the separation of solute from solvent occurs. In most cases, the membrane is designed to allow only solvent (such as water) to pass through this dense layer, while preventing the passage of solutes (such as salt ions). As discussed in U.S. Pat. No. 4,842,736, spiral wound cartridges include adhesives to seal the interstices or voids in the membrane, which prevents leakage of feed fluid to the permeate collection tube. In a particular example, as discussed in U.S. Pat. No. 7,303,675, the adhesive can be applied to an upstream surface of a membrane material in locations where a downstream surface has been treated that flow therethrough is prevented. The adhesive can be applied in other locations of the membrane material as required by the structure and/or application the membrane is utilized in, such as, for example, folds of the membrane, along longitudinal edges, a downstream surface, and the like.
Since the cartridge is subjected to high water pressure, the fully-cured adhesive must be strong and durable. If the hardness of the fully-cured adhesive is too low, then the pressure of the water can cause failure of the adhesive bond, and ultimately the failure of the cartridge. However, if the hardness of the fully-cured adhesive is too high then the adhesive can become brittle and also cause failure. The hardness of the adhesive can be measured on the Shore hardness scale, and the methods of taking such measurements are known in the art. Further, it is noted that the tensile strength of a fully-cured adhesive is used as a measure of suitability of the adhesive in a reverse osmosis module. One skilled in the art will appreciate that a further characteristic of a successful adhesive is the ability of the adhesive to penetrate the membrane material in order to establish a sufficient adhesive bond.
As noted in U.S. Pat. No. 7,303,675, it has been found that over time, and through use of cleaning solutions, traditionally used adhesives may crack or blister (often referred to as “osmotic blistering”), which results in side seal leakage and end seal leakage (often referred to as “veining” or “lightning bolt” failure) of the feed fluid to the permeate collection tube. Osmotic blistering may also encourage bacterial growth and may also cause dimensional change of the membrane, making it difficult to remove and replace. Osmotic blistering is exacerbated by the use of alkaline cleaning fluids, such as, for example, sodium hydroxide solution, which are frequently used in the industry. Thus, there is a need for an improved adhesive composition that also has an increased or improved resistance to alkaline fluids while retaining the performance requirements noted above.
The present invention is believed to be an answer to the need of providing an adhesive that is durable in reverse osmosis applications and is resistant to alkaline fluids.